system/boot: Add support for multiple bootloaders

ARM: define bootloader stack in linker script

This removes the need for the whole HAIKU_BOARD_LOADER_STACK_BASE
sing and dance, since it is always included in the bootloader binary
itself.

More work towards hybrid support

* All packaging architecture dependent variables do now have a
respective suffix and are set up for each configured packaging
architecture, save for the kernel and boot loader variables, which
are still only set up for the primary architecture.
For convenience TARGET_PACKAGING_ARCH, TARGET_ARCH, TARGET_LIBSUPC++,
and TARGET_LIBSTDC++ are set to the respective values for the primary
packaging architecture by default.
* Introduce a set of MultiArch* rules to help with building targets for
multiple packaging architectures. Generally the respective targets are
(additionally) gristed with the packaging architecture. For libraries
the additional grist is usually omitted for the primary architecture
(e.g. libroot.so and <x86>libroot.so for x86_gcc2/x86 hybrid), so that
Jamfiles for targets built only for the primary architecture don't
need to be changed.
* Add multi-arch build support for all targets needed for the stage 1
cross devel package as well as for libbe (untested).

PPC: Comment out cpu-specific compile flags for Sam460ex

Userland and even kernel shouldn't require this anyway,
cpu-specific code can still be compiled with those flags separately.

Sam460ex: Change load address for debugging

* Change haiku_loader load address to leave U-Boot's exceptions vector for easier debugging.

Sam460ex: force using hardware floating point

* our current gcc can't be built with multilib for ppc anyway,
* this allows going further on real hardware, though dprintf() sends wrong data to the serial port.

Fix passing flags from BoardSetup

* since we include BoardSetup earlier now, the TARGET_* flags were discarded. Use HAIKU_* instead.
* Add variables to hold the default entry point and the desired uimage OS emulation.

Sam460ex: Add a Linux-type kernel entry point

* the onboard U-Booot and 2nd-stage loader only know a few OS types,
we'll try faking Linux there.

No need to force TARGET_BOOT_PLATFORM now.

Add board definition for ACube Sam460ex

* It's a PowerPC embedded board that uses U-Boot.
* We don't want an SD image (yet?)
* tune for the ppc440 with soft-float for now.

ARM: define bootloader stack in linker script

This removes the need for the whole HAIKU_BOARD_LOADER_STACK_BASE
sing and dance, since it is always included in the bootloader binary
itself.

More work towards hybrid support

* All packaging architecture dependent variables do now have a
respective suffix and are set up for each configured packaging
architecture, save for the kernel and boot loader variables, which
are still only set up for the primary architecture.
For convenience TARGET_PACKAGING_ARCH, TARGET_ARCH, TARGET_LIBSUPC++,
and TARGET_LIBSTDC++ are set to the respective values for the primary
packaging architecture by default.
* Introduce a set of MultiArch* rules to help with building targets for
multiple packaging architectures. Generally the respective targets are
(additionally) gristed with the packaging architecture. For libraries
the additional grist is usually omitted for the primary architecture
(e.g. libroot.so and <x86>libroot.so for x86_gcc2/x86 hybrid), so that
Jamfiles for targets built only for the primary architecture don't
need to be changed.
* Add multi-arch build support for all targets needed for the stage 1
cross devel package as well as for libbe (untested).

PPC: Comment out cpu-specific compile flags for Sam460ex

Userland and even kernel shouldn't require this anyway,
cpu-specific code can still be compiled with those flags separately.

Sam460ex: Change load address for debugging

* Change haiku_loader load address to leave U-Boot's exceptions vector for easier debugging.

Sam460ex: force using hardware floating point

* our current gcc can't be built with multilib for ppc anyway,
* this allows going further on real hardware, though dprintf() sends wrong data to the serial port.

Fix passing flags from BoardSetup

* since we include BoardSetup earlier now, the TARGET_* flags were discarded. Use HAIKU_* instead.
* Add variables to hold the default entry point and the desired uimage OS emulation.

Sam460ex: Add a Linux-type kernel entry point

* the onboard U-Booot and 2nd-stage loader only know a few OS types,
we'll try faking Linux there.

No need to force TARGET_BOOT_PLATFORM now.

Add board definition for ACube Sam460ex

* It's a PowerPC embedded board that uses U-Boot.
* We don't want an SD image (yet?)
* tune for the ppc440 with soft-float for now.